A wide variety of potting soils and other growing medium have been developed for growing potted plants. Perhaps the most common are mixtures having a base of sphagnum peat moss, soil, ground coconut coir, composted hardwood bark, or composted manure. Soilless potting mixes are desirable because quality top soil can be hard to find, and in any event it can be inconsistent in terms of air and water circulation and can also contain diseased organisms. Achieving the best growing results requires balance in terms of chemistry, composition and structure of the soil or the cultivation medium.
Many characteristics of engineered soilless cultivation medium can vary and have significant effect on the growing response of the plants. For example, the cation exchange capacity of the cultivation medium can vary substantially, or the major minerals in the cultivation medium can be out of balance, or key minor or trace minerals can be nonexistent. It has been said that the availability of the most abundant nutrient in a soil is only as good as the availability of the least abundant nutrients in the soil. For example, if calcium is deficient, many nutrients cannot be transported to and adsorbed by the roots. On the other hand, an excess of certain minerals or nutrients can block the plants ability to adsorb other vital nutrients. For example, if a soil or cultivation medium has too much potassium, the potassium can block the absorption of magnesium. Too much phosphorus can block zinc and calcium uptake by precipitating phosphorus compounds low in solubility. There are also more complex relationships: such as, when the combination of the concentrations of sodium and potassium is excessive, magnesium uptake is blocked even when there is an appropriate amount of magnesium available.
Beginning in the 1930s, William Albrecht from the University of Missouri came to the conclusion that the ideal balance of major cations in agricultural soil should be balanced in view of the cation exchange capacity (CEC) of the soil. Cation exchange capacity is the maximum quantity of exchangeable cations that a soil is capable of holding. Cation exchange capacity in this regard is a measure of fertility and nutrient retention capacity. It is expressed in units of milliequivalent of hydrogen per 100 g of dry soil (mEq/100 g), and corresponds to the number of positive charges that the soil can hold. Closely related to the cation exchange capacity is the concept of base saturation percentage. Base saturation percentage is the fraction of exchangeable cations of the major cation nutrients in the soil (Ca++, Mg++, K+, Na+ and H+).
In common soils, clay particles and humus (organic matter) carry negative charge sites. Fertilizer components having a positive charge (i.e. cations) are attracted to these negative sites and are thus held in the soil to be exchanged with plant roots. In general, the higher the clay content, the higher ability of the soil to hold cations and the higher the cation exchange capacity. In comparison, very sandy soil does not have the ability to hold and exchange nutrients. It is therefore more difficult to grow healthy food crops in sandy soil because of leaching and the inability to hold vital nutrients in place for exchange and uptake into the plant root. Yet, soil with too much clay and not enough sand and other matter will tend to compact and have insufficient air pore space and water pore space.
The cation exchange capacity (CEC) is calculated in soils through the summation of cations found in soil extraction solutions. While there are various types of soil extraction tests used in the art, the Mehlich III test is a standard soil extraction test used by many in the art. In agricultural applications, cation exchange capacity is sometimes measured with the goal of determining the appropriate amounts of mineral additives necessary to achieve balanced base cation saturation ratios (BCSR). Neal Kinsey and Charles Walters authored a book entitled “Hands on Agronomy”, Acres U.S.A., Copyright 1993-2009, which built on Williams Albrecht's work and has become one of the most widely known and influential works on the BCSR system. Kinsey et al. have defined the modem ideal ratios of 60-70% calcium (Ca++), 10-20% magnesium (Mg++), 3-5% potassium (K+), 1% sodium (Na+), and 10-15% exchange hydrogen (H+), and 2-4% other cations. See, e.g., Kinsey et al., “Hands on Agronomy”, pp. 25, 30-31, 34, 49-53, 80 and 96. In agricultural applications, it is normally prohibitively expensive to amend the soil to any large degree in order to provide an optimum cation exchange capacity. Rather, fertilizers are added in order to balance the cation saturations ratios in order to utilize to the soil's natural cation exchange capacity to its fullest extent.
It is generally thought desirable for the cation exchange capacity to be between 10-18 mEq/100 g (soil) in order for the soil to optimally hold and store sufficient nutrients, and then release them appropriately to the plant as the plant needs the nutrients. According to the Albrecht method, the soil should also contain organic matter (humus) in the range of about 5-10% (by volume) in order to assist in providing sufficient nitrogen. Nitrogen in organic matter, namely humus, is released effectively when soil chemistry and physics are balanced to ideal levels. In addition, soil bacteria converts nitrogen from air and may provide up to an additional 35% of nitrogen if soil conditions are again balanced to ideal levels. According to Albrecht, it is highly desirable that at least seventeen (17) other major nutrients, minor nutrients and trace elements must be present, in addition to the major cation ratios being balanced, in order to foster optimum plant growth and nutrient uptake. BCSR practitioners believe that adhering to the method shortens growth times, increases yields, and provides more nutrient dense crops. Also, healthier plants and shorter growing cycles limits susceptibility to pests and weeds. Many professional agronomists conduct soil audits and practice the BCSR system for small to large scale farming operations.
U.S. Pat. No. 4,168,962 entitled “Plant Growth Medium” by Victor N. Lambeth assigned to The Curators of the University of Missouri, issuing on Sep. 25, 1979 discloses a foam-like composition of vermiculite, perlite and clay that is used as a potting soil substitute for container grown plants. The '962 patent discloses the addition of cations based on a recommended percentage saturation of the subsoil exchange capacity (although somewhat different than the modem ideal ratios published by Neal Kinsey et al.). In the '962 patent, a slurry of nutrient rich clay, vermiculite and perlite was mixed and dried to make a rigid foam-like inorganic growth medium. Other nutrients, apart from the nutrients naturally in the clay, were not added. The '962 patent compares growth results of tomatoes in the inorganic growth medium to the growth of tomatoes in Cornell peat-lite mix A (which is believed to have been a mix of peat moss, vermiculite, limestone, phosphate, calcium and potassium). The Cornell peat-lite A mixture, however, was not balanced per the BCSR method, and as with the foam-like plant inorganic plant growth medium, was not tested for other minerals and trace elements.